1. Field of the Invention
This invention relates to a tachometer for use in a disc pack servo system and more particularly to a tachometer that produces an output signal having a magnitude proportional to the average velocity of radial movement of a servo head in such system.
2. Description of the Prior Art
In disc data storage systems, data is stored on concentric tracks on a magnetic disc surface. In order accurately and quickly to access the data stored on a prescribed track, it is imperative that the speed of radial movement of the magnetic heads be measured continuously and accurately. This is recognized in the known prior art and various tachometers for producing velocity information are disclosed.
Among the known tachometer systems are those utilizing an independent transducer such as a magnetic transducer, those using an independent position transducer such as an optical system (e.g. U.S. Pat. No. 3,568,059 to Sordello and U.S. Pat. No. 3,811,091 to Ha et al.), those deriving a position signal from the disc pack and differentiating that signal with respect to time (e.g. U.S. Pat. No. 3,534,344 to Santana), those integrating the motor current which is proportional to acceleration (e.g. Sordello, supra), and those employing two or more of the above enumerated expedients in combination. These prior art tachometers employ analog circuits to perform the velocity calculation. Not only are sophisticated analog circuits complex, but they are also limited in frequency response. The limitation of frequency response increases in importance as advancements in disc technology make possible greater and greater densities of data storage.